Light source apparatus and its control method

ABSTRACT

A light source device can attain a stable output of a harmonic even when there occurs a change in the ambient temperature or fluctuation in the output power. The light source device is provided with a semiconductor laser source ( 4 ), an optical waveguide-type QPM-SHG device ( 5 ) for generating a second harmonic from light emitted from the semiconductor laser source ( 4 ), a wavelength control means ( 7 ) for controlling a wavelength of light emitted from the semiconductor laser source ( 4 ), a means for slightly fluctuating wavelength ( 8 ) for changing a wavelength of light emitted from the semiconductor laser source ( 4 ) and a means for detecting a change in output light power of the optical waveguide-type QPM-SHG device ( 5 ) that occurs when a wavelength of light emitted from the semiconductor laser source ( 4 ) is changed. In this case, a wavelength of light emitted from the semiconductor laser source ( 4 ) is controlled to an optimum wavelength of the optical waveguide-type QPM-SHG device ( 5 ) based on a change in output light power of the optical waveguide-type QPM-SHG device ( 5 ) that occurs when a wavelength of light emitted from the semiconductor laser source ( 4 ) is changed.

TECHNICAL FIELD

The present invention relates to light source devices and methods forcontrolling the same. More specifically, the present invention relatesto a short-wavelength light source that includes a semiconductor laserdevice and a second harmonic generation device, as well as to the methodfor controlling the output light intensity of the same.

BACKGROUND ART

Compact short-wavelength light sources are required in order to achievehigh density for optical disks and high resolution for displays.Coherent light sources using a semiconductor laser and a quasi phasematching (referred to as “QPM” in the following) optical waveguide-typesecond harmonic generation (referred to as “SHG” in the following)device (optical waveguide-type QPM-SHG device) have received attentionas compact short-wavelength light sources (see Yamamoto et al., OpticsLetters, Vol. 16, No. 15, 1156 (1991)).

FIG. 20 shows a diagram showing the configuration of an SHG blue lightsource using an optical waveguide-type QPM-SHG device. As shown in FIG.20, a wavelength-variable distributed Bragg reflector (referred to as“DBR” in the following) semiconductor laser 54 having a DBR region isused as the semiconductor laser. The wavelength-variable DBRsemiconductor laser 54 is a 0.85 μm band 100 mW level AlGaAswavelength-variable DBR semiconductor laser, and includes an activelayer region 56, a phase adjustment region 57 and a DBR region 58. Theoscillation wavelength can be changed continuously by controlling thecurrents injected into the phase adjustment region 57 and the DBR region58 at a constant ratio.

The optical waveguide-type QPM-SHG device 55 serving as the secondharmonic generation device is made of an optical waveguide 60 andperiodic polarity inversion regions 61 formed on an X-cut MgO-dopedLiNbO₃ substrate 59. The optical waveguide 60 is formed by protonexchange in pyrophosphoric acid. Moreover, the periodic polarityinversion regions 61 are fabricated by forming a comb-shaped electrodeon the X-cut MgO-doped LiNbO₃ substrate 59 and applying an electricfield.

In the SHG blue light source shown in FIG. 20, 75 mW of laser light arecoupled into the optical waveguide 60 for 100 mW of the laser output. Bycontrolling the amounts of current injected into the phase adjustmentregion 57 and the DBR region 58 of the wavelength-variable DBRsemiconductor laser 54, the oscillation wavelength is fixed within thephase matching wavelength tolerance width of the optical waveguide-typeQPM-SHG device 55. The use of this SHG blue light source provides about25 mW of blue light of 425 nm wavelength, and the obtained blue light,when lateral mode is the TE₀₀ mode, has focusing property of thediffraction limit and also low noise performance with a relative noisefield intensity of −140 dB/Hz or less, which are characteristicssuitable for reproducing optical disks.

When the optical waveguide-type QPM-SHG device serving as the secondharmonic generation device is evaluated for the output characteristicsof blue light with respect to the wavelength of the fundamental wave, itcan be seen that its wavelength width at which the output of blue lightis half (wavelength tolerance width for phase matching) is as small asabout 0.1 nm. This presents a significant problem in obtaining a stableoutput of blue light. In order to solve this problem, conventionally,the wavelength-variable DBR semiconductor laser is used as thefundamental wave and the wavelength (oscillation wavelength) of thefundamental wave is fixed within the phase matching wavelength tolerancewidth of the optical waveguide-type QPM-SHG device, thereby realizing astable output of blue light.

Ordinarily, the oscillation wavelength of the semiconductor laser sourcechanges with the ambient temperature, and the optimum wavelength of theoptical waveguide-type QPM-SHG device also changes with the ambienttemperature. Therefore, conventionally, the temperatures of thesemiconductor laser source and the optical waveguide-type QPM-SHG deviceare maintained constant by using a Peltier device or the like, therebystabilizing the output of blue light.

However, when considering the installation in optical informationprocessing equipment, such as optical disk devices and laser printers,the average output power changes every instant during operation. In thiscase, even when the ambient temperature is maintained constant by usinga Peltier device or the like, the amount of heat generated by thesemiconductor laser source changes, so that the temperature of thesemiconductor laser source itself changes and hence the oscillationwavelength changes, making it impossible to obtain a stable output ofblue light.

Moreover, in the case of omitting the use of a temperature controldevice such as a Peltier device for the purpose of downsizing thedevice, the fluctuation in the ambient temperature further increases,leading to a change in the output of the optical waveguide-type QPM-SHGdevice.

DISCLOSURE OF INVENTION

The present invention was achieved in order to solve the foregoingproblems in the prior art, and it is an object of the present inventionto provide a light source device that includes a semiconductor lasersource and a second harmonic generation device and that can attain astable output of a harmonic even when there occurs a change in theambient temperature or fluctuation in the output power. Moreover, it isanother object of the present invention to provide a control method thatcan realize such a light source device.

In order to achieve the above-described objects, a first configurationof a light source device in accordance with the present inventionincludes: a semiconductor laser source; a second harmonic generationdevice for generating a second harmonic from light emitted from thesemiconductor laser source; a means for controlling a wavelength oflight emitted from the semiconductor laser source; a means for changinga wavelength of light emitted from the semiconductor laser source; and ameans for detecting a change in output light power of the secondharmonic generation device that occurs when the wavelength of lightemitted from the semiconductor laser source is changed, wherein thewavelength of light emitted from the semiconductor laser source iscontrolled to an optimum wavelength of the second harmonic generationdevice based on the change in output light power of the second harmonicgeneration device that occurs when the wavelength of light emitted fromthe semiconductor laser source is changed.

A second configuration of a light source device in accordance with thepresent invention includes: a semiconductor laser source; a secondharmonic generation device for generating a second harmonic from lightemitted from the semiconductor laser source; a means for controlling anoptimum wavelength of the second harmonic generation device; a means forchanging a wavelength of light emitted from the semiconductor lasersource; and a means for detecting a change in output light power of thesecond harmonic generation device that occurs when the wavelength oflight emitted from the semiconductor laser source is changed, wherein awavelength of the second harmonic generation device is controlled to theoptimum wavelength based on the change in output light power of thesecond harmonic generation device that occurs when the wavelength oflight emitted from the semiconductor laser source is changed.

A third configuration of a light source device in accordance with thepresent invention includes: a semiconductor laser source; a secondharmonic generation device for generating a second harmonic from lightemitted from the semiconductor laser source; a means for controllingoutput light power of the semiconductor laser source in such a mannerthat a power of the second harmonic emitted from the second harmonicgeneration device is constant; a means for controlling a wavelength oflight emitted from the semiconductor laser source; a means for changingthe wavelength of light emitted from the semiconductor laser source; anda means for detecting a change in output light power of thesemiconductor laser source or a change in an amount of a currentinjected into the semiconductor laser source that occurs when thewavelength of light emitted from the semiconductor laser source ischanged, wherein the wavelength of light emitted from the semiconductorlaser source is controlled to an optimum wavelength of the secondharmonic generation device based on the change in output light power ofthe semiconductor laser source or the change in the amount of thecurrent injected into the semiconductor laser source that occurs whenthe wavelength of light emitted from the semiconductor laser source ischanged.

A fourth configuration of a light source device in accordance with thepresent invention includes: a semiconductor laser source; a secondharmonic generation device for generating a second harmonic from lightemitted from the semiconductor laser source; a means for controllingoutput light power of the semiconductor laser source in such a mannerthat a power of the second harmonic emitted from the second harmonicgeneration device is constant; a means for controlling an optimumwavelength of the second harmonic generation device; a means forchanging a wavelength of light emitted from the semiconductor lasersource; and a means for detecting a change in output light power of thesemiconductor laser source or a change in an amount of a currentinjected into the semiconductor laser source that occurs when thewavelength of light emitted from the semiconductor laser source ischanged, wherein a wavelength of the second harmonic generation deviceis controlled to the optimum wavelength based on the change in outputlight power of the semiconductor laser source or the change in theamount of the current injected into the semiconductor laser source thatoccurs when the wavelength of the semiconductor laser source is changed.

A first method for controlling a light source device in accordance withthe present invention is a method for controlling a light source deviceincluding a semiconductor laser source and a second harmonic generationdevice for generating a second harmonic from light emitted from thesemiconductor laser source, the method including: using at least a meansfor controlling a wavelength of light emitted from the semiconductorlaser source, a means for changing the wavelength of light emitted fromthe semiconductor laser source and a means for detecting a change inoutput light power of the second harmonic generation device that occurswhen the wavelength of light emitted from the semiconductor laser sourceis changed; and controlling the wavelength of light emitted from thesemiconductor laser source to an optimum wavelength of the secondharmonic generation device based on the change in output light power ofthe second harmonic generation device that occurs when the wavelength oflight emitted from the semiconductor laser source is changed.

A second method for controlling a light source device in accordance withthe present invention is a method for controlling a light source deviceincluding a semiconductor laser source and a second harmonic generationdevice for generating a second harmonic from light emitted from thesemiconductor laser source, the method including: using at least a meansfor controlling an optimum wavelength of the second harmonic generationdevice, a means for changing a wavelength of light emitted from thesemiconductor laser source, a means for detecting a change in outputlight power of the second harmonic generation device that occurs whenthe wavelength of light emitted from the semiconductor laser source ischanged; and controlling a wavelength of the second harmonic generationdevice to the optimum wavelength based on the change in output lightpower of the second harmonic generation device that occurs when thewavelength of light emitted from the semiconductor laser source ischanged.

A third method for controlling a light source device in accordance withthe present invention is a method for controlling a light source deviceincluding a semiconductor laser source and a second harmonic generationdevice for generating a second harmonic from light emitted from thesemiconductor laser source, the method including: using at least a meansfor controlling output light power of the semiconductor laser source insuch a manner that a power of the second harmonic emitted from thesecond harmonic generation device is constant, a means for controlling awavelength of light emitted from the semiconductor laser source, a meansfor changing the wavelength of light emitted from the semiconductorlaser source and a means for detecting a change in output light power ofthe semiconductor laser source or a change in an amount of a currentinjected into the semiconductor laser source that occurs when thewavelength of light emitted from the semiconductor laser source ischanged; and controlling the wavelength of the semiconductor lasersource to an optimum wavelength of the second harmonic generation devicebased on the change in output light power of the semiconductor lasersource or the change in the amount of the current injected into thesemiconductor laser source that occurs when the wavelength of thesemiconductor laser source is changed.

A fourth method for controlling a light source device in accordance withthe present invention is a method for controlling a light source deviceincluding a semiconductor laser source and a second harmonic generationdevice for generating a second harmonic from light emitted from thesemiconductor laser source, the method including: using at least a meansfor controlling output light power of the semiconductor laser source insuch a manner that a power of the second harmonic emitted from thesecond harmonic generation device is constant, a means for controllingan optimum wavelength of the second harmonic generation device, a meansfor changing a wavelength of light emitted from the semiconductor lasersource and a means for detecting a change in output light power of thesemiconductor laser source or a change in an amount of a currentinjected into the semiconductor laser source that occurs when thewavelength of light emitted from the semiconductor laser source ischanged; and controlling a wavelength of the second harmonic generationdevice to the optimum wavelength based on the change in output lightpower of the semiconductor laser source or the change in the amount ofthe current injected into the semiconductor laser source that occurswhen the wavelength of light emitted from the semiconductor laser sourceis changed.

A fifth configuration of a light source device in accordance with thepresent invention includes: a semiconductor laser source including atleast an active layer region and a phase adjustment region; a means forinjecting into the phase adjustment region a current that is inanti-phase with a current injected into the active layer region whenmodulation is performed by switching an average output power of thesemiconductor laser source between at least two values corresponding toa low output and a high output, respectively; and a means forasymptotically changing a current injected into the phase adjustmentregion after the switching, starting immediately after the switching.

A sixth configuration of a light source device in accordance with thepresent invention includes: a semiconductor laser source including atleast an active layer region, a phase adjustment region and adistributed Bragg reflector (DBR) region; a means for injecting into thephase adjustment region and the DBR region a current that is inanti-phase with a current injected into the active layer region whenmodulation is performed by switching an average output power of thesemiconductor laser source between at least two values corresponding toa low output and a high output, respectively; and a means forasymptotically changing the current injected into the phase adjustmentregion and the DBR region after the switching, starting immediatelyafter the switching.

A seventh configuration of a light source device in accordance with thepresent invention includes a semiconductor laser source including atleast an active layer region and a phase adjustment region; and a meansfor driving the phase adjustment region, using a signal obtained bypassing through a filter a drive signal of the active layer region whenmodulation is performed by switching an average output power of thesemiconductor laser source between at least two values corresponding toa low output and a high output, respectively.

An eighth configuration of a light source device in accordance with thepresent invention includes a semiconductor laser source including atleast an active layer region and a phase adjustment region; a secondharmonic generation device for generating a second harmonic from lightemitted from the semiconductor laser source; a means for injecting intothe phase adjustment region a current that is in anti-phase with acurrent injected into the active layer region when modulation isperformed by switching an average output power of the semiconductorlaser source between at least two values corresponding to a low outputand a high output, respectively; and a means for controlling the currentinjected into the active layer region in such a manner that a power ofthe second harmonic generated in the second harmonic generation devicebecomes constant after the switching.

A ninth configuration of a light source device in accordance with thepresent invention includes a semiconductor laser source including atleast an active layer region and a phase adjustment region; a secondharmonic generation device for generating a second harmonic from lightemitted from the semiconductor laser source; a means for injecting intothe phase adjustment region a current that is in anti-phase with acurrent injected into the active layer region when modulation isperformed by switching an average output power of the semiconductorlaser source between at least two values corresponding to a low outputand a high output, respectively; and a means for controlling the currentinjected into the active layer region and the phase control region insuch a manner that a power of the second harmonic generated in thesecond harmonic generation device becomes constant after the switching.

A configuration of an optical information recording/reproducingapparatus in accordance with the present invention includes: theabove-described light source device; a condensing optical system forguiding light from the light source device to an information carrier;and a means for detecting light reflected from the information carrier.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the configuration of a light source deviceaccording to a first embodiment of the present invention,

FIG. 2A is a graph showing the output power of an emitted harmonic withrespect to the wavelength of a fundamental wave in the first embodiment,and FIG. 2B is a graph showing how the output power of blue lightfluctuates in the same embodiment,

FIG. 3 is a diagram showing the configuration of a light source deviceaccording to a second embodiment of the present invention,

FIG. 4A is a graph showing the output power of an emitted harmonic withrespect to the wavelength of a fundamental wave in the secondembodiment, and FIG. 4B is a graph showing the relation between thechange of the amount of injection current over time and the slightfluctuation in the wavelength in the same embodiment,

FIG. 5 is a diagram showing the configuration of another example of alight source device according to the second embodiment of the presentinvention,

FIG. 6 is a diagram showing the configuration of a light source deviceaccording to a third embodiment of the present invention,

FIG. 7 is a perspective view showing how a thin film heater is installedin the third embodiment of the present invention,

FIG. 8 is a graph conceptually showing the waveform of an output lightfluctuation caused when switching between a reproducing operation and arecording operation, that is, when the average output power of the lightsource is switched from a low output state to a high output state in anoptical disk device in a fourth embodiment of the present invention,

FIG. 9 is a graph conceptually showing how the output power of aharmonic and the wavelength of the fundamental wave fluctuate whenswitching the output power in the fourth embodiment of the presentinvention,

FIG. 10 is a schematic sectional view showing a semiconductor lightsource according to the fourth embodiment of the present invention,

FIG. 11 is a graph showing the complementary drive waveforms whenswitching between recording and reproducing in the fourth embodiment ofthe present invention (FIG. 11A is a graph showing the temporal changeof the currents injected into an active layer region and a phaseadjustment region and FIG. 11B is a graph showing the change of therefractive indexes of the active layer region over time and the phaseadjustment region at that time),

FIG. 12 is a graph showing the drive currents when switching betweenrecording and reproducing used for suppressing the fluctuation in theoutput of a harmonic due to the fluctuation in an oscillation wavelengthin the fourth embodiment of the present invention (FIG. 12A is a graphshowing the change of the currents injected into an active layer regionover time and a phase adjustment region and FIG. 12B is a graph showingthe temporal change of the refractive indexes of the active layer regionand the phase adjustment region at that time),

FIG. 13 is a block diagram showing a circuit configuration for providingthe current injected into a phase adjustment region shown in FIG. 12 ina fifth embodiment of the present invention,

FIG. 14 is a block diagram showing another circuit configuration forachieving the current injected into the phase adjustment region shown inFIG. 12 in the fifth embodiment of the present invention,

FIG. 15 is a block diagram showing the control circuit of a light sourcedevice according to a six embodiment of the present invention,

FIGS. 16A to C are diagrams showing the temporal change of the currentinjected into an emission portion of a light source device according tothe sixth embodiment of the present invention,

FIG. 17 is a block diagram showing the control circuit of a light sourcedevice according to a seventh embodiment of the present invention,

FIG. 18 is a diagram showing the waveforms of signals used in theseventh embodiment of the present invention,

FIG. 19 is a conceptual diagram showing a band-pass filter used as anequalization means in the seventh embodiment of the present invention,and

FIG. 20 is a diagram showing the configuration of an SHG blue lightsource using an optical waveguide-type QPM-SHG device according to theprior art.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, the present invention will be described morespecifically with reference to embodiments.

First Embodiment

FIG. 1 is a diagram showing the configuration of a light source deviceaccording to a first embodiment of the present invention.

As shown in FIG. 1, in the light source device of this embodiment, a0.85 μm band 100 mW level AlGaAs wavelength-variable distributed Braggreflector (referred to as “DBR” in the following) semiconductor lasersource 4 that includes a DBR region 1, a phase adjustment region 2 foradjusting the phase of light in the laser with an injection current andan active layer region 3 for controlling the output power with aninjection current, is used as the semiconductor laser source used for afundamental wave.

Moreover, a quasi phase matching (referred to as “QPM” in the following)optical waveguide-type second harmonic generation (referred to as “SHG”in the following) device (optical waveguide-type QPM-SHG device) 5 isused as a second harmonic generation device. More specifically, theoptical waveguide-type QPM-SHG device 5 is made of an optical waveguide12 and periodic polarity inversion regions that are arrangedperpendicular to the optical waveguide 12 for compensating thepropagation constant difference between the fundamental wave andharmonics, formed on the upper surface of an optical crystal substrate(0.5 mm thick X-cut MgO-doped LiNbO₃ substrate) 11 using lithium niobate(LiNbO₃). The optical waveguide 12 is formed by proton exchange inpyrophosphoric acid. Moreover, the periodic polarity inversion regionsare fabricated by forming a comb-shaped electrode on the X-cut MgO-dopedLiNbO₃ substrate 11 and applying an electric field. With the opticalwaveguide-type QPM-SHG device 5, it is possible to realize a highconversion efficiency, because it is possible to utilize its largenonlinear optical constants, and also because it is of the opticalwaveguide type and a long interaction length can be established.

The semiconductor laser source 4 and the optical waveguide-type QPM-SHGdevice 5 are integrated on a Si submount 6, and their temperature iscontrolled with a Peltier device. The semiconductor laser light servingas the fundamental wave is coupled into the optical waveguide of theoptical waveguide-type QPM-SHG device 5 by direct coupling, withoutusing a lens. That is, a fundamental wave emitted from the semiconductorlaser source 4 is made incident on the optical waveguide-type QPM-SHGdevice 5, and the fundamental wave incident on the opticalwaveguide-type QPM-SHG device 5 is trapped inside the optical waveguide12 and propagates therethrough. The fundamental wave propagating throughthe optical waveguide 12 is converted into a second harmonic by thenonlinearity of the optical crystal (X-cut MgO-doped LiNbO₃), and aharmonic whose wavelength is one half that of the fundamental wave isemitted from the emission-side end face of the optical waveguide-typeQPM-SHG device 5.

Due to the chromatic dispersion of the optical crystal (X-cut MgO-dopedLiNbO₃), the optical waveguide-type QPM-SHG device (also referred to as“SHG device” in the following) 5 having the above-describedconfiguration has wavelength characteristics as shown in FIG. 2A withrespect to the wavelength of the incident fundamental wave. FIG. 2Ashows the output power of the emitted harmonic with respect to thewavelength of the incident fundamental wave. The harmonic exhibits theoutput characteristics expressed by a SINC function as shown in thefollowing Equation 1 with respect to the wavelength λ of the fundamentalwave, taking the optimum wavelength λ0 of the fundamental wave as thepeak. $\begin{matrix}\begin{matrix}{y = {{Sinc}\left\{ {\left( {\lambda - {\lambda 0}} \right) \times {\pi/a}} \right\}}} \\{= {\sin{\left\{ {\left( {\lambda - {\lambda 0}} \right) \times {\pi/a}} \right\}/\left\{ {\left( {\lambda - {\lambda 0}} \right) \times {\pi/a}} \right\}}}}\end{matrix} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

Here, the wavelength tolerance, which is represented by the wavelengthwidth at which the output power of the harmonic is one half the maximumvalue, {hacek over (E)}0.1 nm, and it is necessary that the wavelengthof the fundamental wave be precisely and stably controlled at λ0 inorder to obtain a stable output of blue light.

The following describes a method for controlling the oscillationwavelength of the semiconductor laser source 4 shown in FIG. 1.

Ordinarily, in a semiconductor laser source, only light with thewavelength λ that satisfies the following Equation 2 with respect to theoptical distance L between the front and rear reflection planes isexcited.2L=nλ(n: integer)  (Equation 2)

A set of the wavelengths λ that satisfy the above Equation 2 is called a“longitudinal mode”, in which case the oscillation wavelengths take ondiscrete values. In the semiconductor laser source 4 shown in FIG. 1, aphase adjustment region 2 is provided between a DBR region 1 and theemission-side end face of the semiconductor laser source 4, and thewavelength λ of the longitudinal mode can be changed by altering theoptical distance L of the semiconductor laser source 4 with the currentapplied to the phase adjustment region 2. Thus, it is possible tocontrol the oscillation wavelength of the semiconductor laser source 4with the current applied to the phase adjustment region 2.

In this wavelength control method, however, the wavelength control rangeis limited for the reason described below. That is, a grating is formedin the DBR region 1 of the semiconductor laser source 4 shown in FIG. 1,and only a light with a wavelength defined by the period of the gratingis reflected. Specifically, when the refractive index of the DBR region1 is n_(dbr) and the grating period of the DBR region 1 is Λ, the rangeof the wavelengths that can be reflected in the DBR region 1 is about2Λ/n_(dbr)±0.1 nm, so that the wavelength control can be performed onlyin this range.

In this embodiment, the following method is adopted in order to broadenthe above-described wavelength control range. That is, an electrode isformed in the DBR region 1, and the current applied to this electrodechanges the effective grating period of the DBR region 1 and also theoptimum wavelength in the DBR region 1. By changing the optimumwavelength in the DBR region 1 in accordance with the change in thewavelength of the longitudinal mode caused by the current applied to thephase adjustment region 2, it is possible to control the oscillationwavelength continuously. In practice, currents with a constant ratio areapplied to the DBR region 1 and the phase adjustment region 2.

Next, a method for stabilizing the output of a light source of thepresent invention is described with reference to FIG. 1.

In FIG. 1, numeral 7 denotes a wavelength control means, and thewavelength control means 7 controls the oscillation wavelength of thesemiconductor laser source 4 to a certain wavelength by controlling thecurrents applied to the DBR region 1 and the phase adjustment region 2.At that time, currents with a constant ratio are applied to the DBRregion 1 and the phase adjustment region 2 of the semiconductor lasersource 4 with a means for periodically changing the wavelength of lightemitted from the semiconductor laser source 8, thereby slightly changingthe oscillation wavelength of the semiconductor laser source 4. Aportion, for example, about 5% of the harmonic output light at this timeis separated with a half mirror, and its change over time is monitoredwith a harmonic output detection means. FIG. 2B shows how the outputpower of blue light fluctuates at that time.

If the oscillation wavelength of the semiconductor laser source 4 iscontrolled in the vicinity of the optimum wavelength λ0 of the opticalwaveguide-type QPM-SHG device 5, the wavelength changes with theoperation point B as the center, and the harmonic output light ismodulated with a period twice that of the wavelength fluctuation signalof the semiconductor laser source 4. At this time, regarding the slightfluctuation in the wavelength, the output of the harmonic when thewavelength is fluctuated to the shorter wavelength side is equal to thatwhen the wavelength is fluctuated to the longer wavelength side.

In contrast, if the oscillation wavelength of the semiconductor lasersource 4 is shifted from the optimum wavelength λ0 of the opticalwaveguide-type QPM-SHG device 5 owing to a change in the ambienttemperature of a change in the drive current of the semiconductor lasersource 4, the operation takes place, for example, with the operationpoint A as the center. FIG. 2B shows a case (the operation point A) inwhich the oscillation wavelength of the semiconductor laser source 4 isshifted to a wavelength shorter than the optimum wavelength λ0 of theoptical waveguide-type QPM-SHG device 5. The harmonic output light atthis time shows an output fluctuation having the same period as thewavelength fluctuation, and the output of the harmonic is larger whenthe wavelength is long, whereas it is smaller when the wavelength isshort. In this way, the phase of a wavelength wobbling signal generatedwith the means for periodically changing the wavelength of light emittedfrom the semiconductor laser source 8 should be compared wit the phaseof the output signal from the harmonic output detection means a phasecomparison means, and when they are in phase as shown in FIG. 2B, thewavelength should be controlled to be longer. Although not shown, if theoscillation wavelength of the semiconductor laser source 4 is shifted toa wavelength longer than the optimum wavelength λ0 of the opticalwaveguide-type QPM-SHG device 5, the output of the harmonic is smallerwhen the wavelength is long, whereas it is larger when the wavelength isshort.

As described above, by monitoring the output power of the harmonic whenthe oscillation wavelength of the semiconductor laser source 4 isslightly changed, it is possible to detect any shift from the optimumwavelength λ0 of the optical waveguide-type QPM-SHG device 5. Then,feeding back this wavelength shift to the oscillation wavelength of thesemiconductor laser source 4 makes it possible to control theoscillation wavelength of the semiconductor laser source 4 continuallyto the optimum wavelength λ0 of the optical waveguide-type QPM-SHGdevice 5 and continually to maintain a constant and maximum conversionefficiency in the optical waveguide-type QPM-SHG device 5 with respectto a constant output of the fundamental wave, thereby stabilizing theoutput of the harmonic.

Second Embodiment

In the wavelength control method described in the first embodiment, theoutput power of the harmonic, although slightly, is changed in order tomonitor the output power of the harmonic. In this embodiment, a methodfor controlling the output power of the harmonic with higher precisionis described. FIG. 3 is a diagram showing the configuration of a lightsource device according to a second embodiment of the present invention.

As shown in FIG. 3, in this embodiment, a harmonic output fixationcontrol means 9 is used in addition to the configuration shown inFIG. 1. More specifically, when slightly fluctuating the oscillationwavelength of the semiconductor laser source 4 with the means forperiodically changing the wavelength of light emitted from thesemiconductor laser source 8, the output of the harmonic is monitoredand fed back, with the harmonic output fixation control means 9, to thecurrent injected into the active layer region 3 of the semiconductorlaser source 4. If the output of the harmonic decreases, the currentinjected into the active layer region 3 of the semiconductor lasersource 4 is increased so as to increase the output of the fundamentalwave, thereby maintaining the output of the harmonic constant. Bysetting the response frequency of this feedback loop sufficiently higherthan the fluctuation frequency of the means for periodically changingthe wavelength of light emitted from the semiconductor laser source 8,the fluctuation amount of the output of the harmonic can be suppressedto be sufficiently low, even when the wavelength is fluctuated.

In this case, shifts of the oscillation wavelength of the semiconductorlaser source 4 from the optimum wavelength of the optical waveguide-typeQPM-SHG device 5 are detected by monitoring the correlation between thesignal of the means for periodically changing the wavelength of lightemitted from the semiconductor laser source 8 and the change of theamount of injection current over time.

FIG. 4B shows the relation between the change of the amount of injectioncurrent over time and the slight fluctuation in the wavelength. Forexample, a signal change is described for the case (the operation pointA in FIG. 4A) that the oscillation wavelength of the semiconductor lasersource 4 is shifted to a wavelength shorter than the optimum wavelengthλ0 of the optical waveguide-type QPM-SHG device 5. When the oscillationwavelength of the semiconductor laser source 4 is made shorter with themeans for periodically changing the wavelength of light emitted from thesemiconductor laser source 8, the conversion efficiency in the opticalwaveguide-type QPM-SHG device 5 decreases. In this case, however, thecurrent injected into the active layer region 3 of the semiconductorlaser source 4 is increased with the harmonic output fixation controlmeans 9 so as to increase the output of the fundamental wave, therebykeeping the output of the harmonic constant. Conversely, when theoscillation wavelength of the semiconductor laser source 4 is madelonger with the means for periodically changing the wavelength of lightemitted from the semiconductor laser soure 8, the current injected intothe active layer region 3 of the semiconductor laser source 4 isdecreased with the harmonic output fixation control means 9. Thus, theoscillation wavelength of the semiconductor laser source 4 is graduallychanged to the longer wavelength side when the change in the injectioncurrent is in anti-phase with respect to the signal of the wavelengthfluctuation, whereas, conversely, the oscillation wavelength of thesemiconductor laser source 4 is gradually changed to the shorterwavelength side when the change in the injection current is in phasewith respect to the signal of the wavelength fluctuation, thereby makingit possible to control the oscillation wavelength of the semiconductorlaser source 4 to the optimum wavelength of the optical waveguide-typeQPM-SHG device 5 and thus stabilizing the output power of the harmonic.

As described above, in combination with the control in which the optimumwavelength is sought by slightly fluctuating the oscillation wavelengthof the semiconductor laser source 4 with the means for periodicallychanging the wavelength of light emitted from the semiconductor lasersource 8, the fixation control of the output of the harmonic isperformed and increases and decreases of the current injected into theactive layer region 3 of the semiconductor laser source 4 are detected,thereby making it possible to suppress the fluctuation in the output ofthe harmonic.

Moreover, a similar effect also can be obtained by detecting anywavelength shift based on the change of the power of the fundamentalwave over time, instead of detecting an increase and decrease of thecurrent injected into the active layer region 3 of the semiconductorlaser source 4. In this case, unlike monitoring the amount of a currentinjected into the active layer region 3 of the semiconductor lasersource 4, no effect is exerted by the change in the emission efficiencyof the semiconductor laser source 4, so that a more precise wavelengthcontrol is possible. This control method can be achieved, as shown inFIG. 5, by arranging a fundamental wave power detection means 10 on theoptical crystal substrate (X-cut MgO-doped LiNbO₃ substrate) 11 that isa part of the optical waveguide-type QPM-SHG device 5 and detecting anylight that has not been coupled into the optical waveguide-type QPM-SHGdevice 5, out of light emitted from the semiconductor laser source 4.

Third Embodiment

The first and second embodiments described light source devices forcontrolling the oscillation wavelength of a semiconductor laser source,but the output power of a harmonic (second harmonic) also can bestabilized by controlling the optimum wavelength of an opticalwaveguide-type QPM-SHG device. FIG. 6 shows the schematic configuration.

As shown in FIG. 6, in the optical waveguide-type QPM-SHG device 5, athin film heater 13 is mounted on the surface of the optical crystalsubstrate (X-cut MgO-doped LiNbO₃ substrate) 11 on the side on which theoptical waveguide 12 is mounted. The thin film heater 13 is formed, forexample, by vapor-depositing aluminum or the like and performingpatterning using a photolithography method. As shown in FIG. 7, the thinfilm heater 13 is formed in a stripe shape along the optical waveguide12, and causes a localized temperature increase at the optical waveguideportion.

Although not shown, in order to reduce a power loss of the opticalwaveguide 12 due to light absorption in the thin film heater 13, abuffer layer made of a dielectric film, of for example silicon dioxide,having a smaller refractive index than the material of the opticalcrystal substrate is provided between the optical waveguide 12 and thethin film heater 13.

When a current is applied to the thin film heater 13, the temperature ofthe optical waveguide portion is increased by Joule heat, causing achange in the refractive index. In this case, since the fundamental waveand the harmonic, each propagating through the optical waveguide 12, aresubjected to different amounts of change in the refractive index, thereoccurs a difference in propagation constant between the fundamental waveand the harmonic, changing the optimum wavelength; accordingly, it ispossible to let the oscillation wavelength of the semiconductor lasersource 4 coincide with the optimum wavelength of the opticalwaveguide-type QPM-SHG device 5. It should be noted that since onlyheating can be performed by the thin film heater 13, a constant amountof current is applied to the thin film heater 13 in the initial emissionstate of the light source, and the current can be increased anddecreased to provide heating and cooling effects.

When controlling the oscillation wavelength of the semiconductor lasersource 4 with the currents injected into the DBR region 1 and the phaseadjustment region 2 as in the method shown in the first or secondembodiment, each of the currents may in some cases increase thetemperature of the semiconductor laser source 4, thereby possiblyreducing the emission efficiency of the semiconductor laser source 4 orshortening the life thereof. In contrast, according to a method forcontrolling the oscillation wavelength of the semiconductor laser source4 by controlling the temperature of the optical waveguide-type QPM-SHGdevice 5 as in this embodiment, it is possible to suppress thetemperature increase of the semiconductor laser source 4, therebyachieving an SHG light source having higher efficiency and longer life.

It should be noted that in this embodiment, the configuration in whichthe thin film heater 13 is provided on the surface of the opticalwaveguide-type QPM-SHG device 5 is described as an example; however, asimilar effect also can be obtained by providing the thin film heater,for example, on the surface of the submount 6 at the portion directlybelow the optical waveguide 12. It is also possible to perform a similarwavelength control with a configuration in which the thin film heater isprovided on the submount 6 at the portion on which the semiconductorlaser source 4 is mounted and the temperature of the semiconductor lasersource 4 is changed to fluctuate the oscillation wavelength of thesemiconductor laser source 4. In this case, the possibility that thetemperature increase may cause degradation in the life characteristicsand emission efficiency characteristics of the semiconductor lasersource 4 is greater than when the oscillation wavelength is controlledwith the amounts of currents injected into the DBR region 1 and thephase adjustment region 2 of the semiconductor laser source 4, but thedegree of the degradation in the life characteristics and emissionefficiency characteristics is negligible because the amount of thecurrent (and hence the current density) in each portion of thesemiconductor laser source 4 is decreased.

Fourth Embodiment

The first to third embodiments described stabilization control withregard to a gradual change of the oscillation wavelength of asemiconductor laser source over time.

The present inventors found that when using a light source of thepresent invention in an optical disk device, there was a characteristicfactor in the wavelength fluctuation caused by modulation operationemployed in the optical disk device. FIG. 8 is a graph conceptuallyshowing the waveform of an output light fluctuation caused whenswitching between a reproducing operation and a recording operation,that is, when the average output power of the light source is switchedfrom a low output state to a high output state in an optical diskdevice.

The inventors observed the waveform of blue output when switchingbetween recording and reproducing, and obtained an output waveform asshown in FIG. 8. That is to say, they found that a gradual decrease inoutput occurred after the switching, even when a high-speed modulationwaveform was attained at the time of recording. The output of thesemiconductor laser source was maintained constant after the switch to ahigh output state, so that it is believed that the decrease in theoutput of the harmonic is due to the change in the oscillationwavelength of the semiconductor laser source.

FIG. 9 is a graph conceptually showing how the output power of harmonicand the wavelength of the fundamental wave fluctuate when switching theoutput power. In FIG. 9, the curves indicated by the solid line and thebroken line denote the dependence of the output power of the harmonic onthe wavelength of the fundamental wave at the time of high output and atthe time of low output, respectively. Although the emission power of thesemiconductor laser source abruptly rises when switching the outputpower, the oscillation wavelength of the semiconductor laser sourceafter switching gradually changes owing to the thermal behavior causedby the change in the injection current, leading to a gradual decrease inthe output of the harmonic. In the following, this phenomenon isdescribed in detail.

FIG. 10 is a schematic sectional view showing a semiconductor lasersource according to the fourth embodiment of the present invention. Atthe time of high-speed modulation, the temperature of the active layerregion 3 changes owing to the change in the current injected into theactive layer region 3, changing the effective optical distance L of thesemiconductor laser source 4. In order to cope with this, the amount ofheat generation in the entire semiconductor laser source 4 can bemaintained substantially constant by applying to the phase adjustmentregion 2 a current that is complementary to the current injected(injection current) into the active layer region 3. At this time, theoptical distance in the active layer region 3 and that in the phaseadjustment region 2 change substantially symmetrically, so that theeffective optical distance L of the semiconductor laser source 4 ismaintained constant. After the recording/reproducing switching, however,the average values of the injection currents fluctuate over a longperiod of time. The currents applied to the active layer region 3 andthe phase adjustment region 2 not only change the temperatures of theirrespective regions, but also cause heat to propagate, as indicated byarrows in FIG. 10, in the thickness direction of the semiconductor lasersource 4 and the direction towards the submount 6 on which thesemiconductor laser source 4 is fixed, thereby changing the ambienttemperature. In this case, since the phase adjustment region 2 and theactive layer region 3 are arranged in different positions, they differwith regard to the temperature increase over time caused by the samecurrent. Therefore, the change in the optical distance in each of theregions falls outside the complementary conditions, leading to thefluctuation in the oscillation wavelength of the semiconductor lasersource 4 and hence the fluctuation in the output power of blue light.

FIG. 11 shows the complementary drive waveforms at the time of theabove-described recording/reproducing switching. As shown in FIG. 11A, adrive current as indicated by the solid line is applied to the activelayer region 3, and a square current as indicated by the broken line isinjected into the phase adjustment region 2 at the same time. At thistime, as shown in FIG. 11B, the changes in the refractive indexes of theactive layer region 3 and the phase adjustment region 2 aresubstantially symmetrical, showing complementary fluctuations. Asdescribed above, since the waveforms of the temperature fluctuation overtime caused in the phase adjustment region 2 and the active layer region3 are different, the total change in the refractive index to which thesemiconductor laser source 4 is subjected shifts from a constant value,as indicated by the dash-dotted line, leading to the fluctuation in theoscillation wavelength.

Therefore, this embodiment achieves a light source device that iscapable of solving the above-described problems and is a blue lightsource made of a wavelength-variable DBR semiconductor laser source andan optical waveguide-type QPM-SHG device, wherein a stable output lightpower can be obtained even after switching the average output power.

In a light source device according to this embodiment, in order tosuppress the fluctuation in the output of the harmonic caused by theabove-described fluctuation in the oscillation wavelength, thedifference in thermal behaviors between the phase adjustment region 2and the active layer region 3 is compensated by injecting into the phaseadjustment region 2 a current that changes asymptotically, after therecording/reproducing switching, as shown in FIG. 12.

A wavelength-variable DBR semiconductor laser source and an opticalwaveguide-type QPM-SHG device were manufactured by way of trial, andmodulation testing was performed. As a result, when the current injectedinto the active layer region 3 was switched from 50 mA to 150 mA, thecurrent injected into the phase adjustment region 2 was switched from100 mA to 45 mA and both of them were driven by a current with a squarewaveform, then, an asymptotic output fluctuation with a time constant of0.15 msec and an amount of fluctuation of −12% was observed in theoutput of the harmonic after the switching. Therefore, like thisembodiment, in addition to the current with a step-like waveform, anasymptotic change over time with an amplitude of −9 mA and a timeconstant of 0.15 msec was added to the current injected into the phaseadjustment region 2 after the switching. Consequently, the fluctuationin the output of the harmonic was suppressed to below the measurementlimit, proving that the control method of the present inventionsuppresses the wavelength fluctuation and thus realizes a stable lightsource device.

It should be noted that although in the examples shown above, thewavelength fluctuation after the switching was compensated only with thecurrent injected into the phase adjustment region 2, the methods forcontrolling the wavelength of the semiconductor laser source alsoinclude a method of using the current injected into the phase adjustmentregion 2 and the current injected into the DBR region 1, and a similareffect also can be obtained by injecting into the phase adjustmentregion 2 and the DBR region 1, current with a constant ratio that showsan asymptotic temporal fluctuation. In this case, compared to the casethat a current showing an asymptotic fluctuation over time is injectedinto only the phase adjustment region 2, it is possible to stabilize thewavelength effectively also for a semiconductor laser source that causeslarger current amplitude (and hence a larger wavelength fluctuation)when switching between recording and reproducing.

Fifth Embodiment

The fourth embodiment described an exemplary case in which the switchingof recording/reproducing is performed merely once and each of thesestates continues for a long period of time. However, when a light sourcedevice of the present invention is used in an actual optical diskdevice, there is the possibility that the recording/reproducingswitching may be repeated at random timings in a short period of time.In this case, the recording/reproducing switching is performed onceagain while the value of the current injected into the phase adjustmentregion or the phase adjustment region and the DBR region is changingasymptotically. In this case, it is effective to use a method ofmodulating the current injected into the phase adjustment region byusing a signal obtained by passing through an appropriate filtercircuit, not only the waveform of a compensation current, but also thewaveform of the current injected into the active layer region. Forexample, in order to achieve the current injected into the phaseadjustment region shown in FIG. 12, a circuit represented by a blockdiagram as shown in FIG. 13 is employed. More specifically, the currentinjected into the phase adjustment region can be approximated by the sumof a signal obtained by reversing the polarity of the waveform of themodulation signal and multiplying it by a constant α and a signalobtained by passing the waveform of the modulation signal through anintegration filter. The circuit characteristics represented by theintegration filter and a constant addition circuit represent thedifference in thermal response characteristics between the phaseadjustment region 2 and the active layer region 3, and it is possible tocause complementary temperature changes in the active layer region 3 andthe phase adjustment region 2 for any arbitrary modulation signal bypassing it through the filter circuit shown in FIG. 13.

In this embodiment, a case was shown in which the component with slowthermal response in the phase adjustment region 2 is smaller than thecomponent with slow thermal response in the active layer region 3, thatis, a case in which the difference in thermal responses can becompensated by gradually increasing the amount of a current injectedinto the phase adjustment region 2 after the recording/reproducingswitching. However, if the semiconductor laser source has a differentconfiguration, for example, if the semiconductor laser source isconfigured by reversing the positional relationship between the phaseadjustment region and the active layer region, it is necessary toperform a correction opposite to that described above. That is, when thecomponent with a slow thermal response in the phase adjustment region islarger than the component with a slow thermal response in the activelayer region, it is necessary to gradually decrease the amount of acurrent injected into the phase adjustment region after therecording/reproducing switching. In this case, it is possible to achievea similar stabilization of the output by using a differentiation filterin place of the integration filter, as in the block diagram shown inFIG. 14.

Sixth Embodiment

As described above, when a light source device made of a semiconductorlaser source and an optical waveguide-type QPM-SHG device is modulated,the output can be stabilized by a wavelength control method ofcompensating a gradual temperature change or the like, and by awavelength fluctuation compensation method of compensating the heatgenerated by the semiconductor laser source at the time of modulation.However, the two methods are more effective when they are simultaneouslyused in an appropriate combination, rather than as separate methods.

In this embodiment, a specific mode of control loops (a fast loop and aslow loop) of the emission portion and wavelength-variable portion of awavelength-variable laser is described particularly with reference toFIGS. 15 and 16. FIG. 15 is a block diagram showing the control circuitof a light source device according to the sixth embodiment of thepresent invention, and FIG. 16 is a diagram showing the change over timeof the current injected into the emission portion of the light sourcedevice of the sixth embodiment.

In FIG. 15, numeral 400 denotes a wavelength-variable laser, and thiswavelength-variable laser 400 is made of an emission portion 401 and awavelength-variable portion 402. Numeral 410 denotes an opticalwaveguide-type QPM-SHG device. A second harmonic emitted from theoptical waveguide-type QPM-SHG device 410 is partly converted into anelectric signal with a spectroscopic device 421 and a photodetector 422,and supplied, via a head amplifier 423, to a feedback loop (fast loop)made of a differential calculation means 424, an integration means 425,a current conversion amplifier 426 and the emission portion 401. Thatis, a current IL injected into the emission portion 401 is controlledsuch that the output signal from the head amplifier 423, which is in aconstant relation with the second harmonic, coincides with the targetvalue. It is desirable that the response frequency of this loop be setto about 100 kHz in view of its relationship with the slow loopdescribed below.

In FIG. 15, the slow loop is made of the spectroscopic device 421, thephotodetector 422, the head amplifier 423, the differential calculationmeans 424, the integration means 425, a band-pass filter 427, amultiplication means 428, a signal source 429, an integration means 430,an addition calculation means 431, a current conversion amplifier 432and the wavelength-variable portion 402. First, a superimposed signal Sdwith a predetermined frequency (about 1 kHz), supplied from the signalsource 429, is converted via the current conversion amplifier 432 into acurrent, which is injected into the wavelength-variable portion 402. Atthis time, the oscillation wavelength of the wavelength-variable laser400 changes corresponding to this signal. When the oscillationwavelength changes, then the conversion efficiency of the opticalwaveguide-type QPM-SHG device 410 changes in accordance with therelation with the wavelength of the maximum efficiency conversion thatis specific to the optical waveguide-type QPM-SHG device 410. If the“fast feedback loop” is not in operation, this change in the conversionefficiency will be directly apparent as the change in the output of thesecond harmonic. However, the response frequency of the fast feedbackloop is 100 kHz, which is sufficiently higher than the frequency of thischange (about 1 kHz), and the decrease in the output of the secondharmonic caused by the decrease in the conversion efficiency isimmediately fed back as the increase in the current IL injected into theemission portion 401, so that a change in the output of the secondharmonic is hardly observed (suppressed to about 1/100 in thisembodiment). However, a feedback signal SL of the feedback current ILinjected into the emission portion 401 changes, as shown in FIGS. 16A, Band C, corresponding to the conversion efficiency of the opticalwaveguide-type QPM-SHG device 410. That is, the feedback signal SL(feedback current IL) is generated so as to cancel the change in theconversion efficiency in the wavelength-variable portion 402 that occursdue to the superimposed disturbance current.

Here, when the central value of the current injected into thewavelength-variable portion 402 is taken as S0, the feedback signal SLchanges to be in phase (FIG. 16C) or in anti-phase (FIG. 16A) with thesignal Sd superimposed thereon, depending on the relation between thiscentral value S0 and the wavelength of the maximum efficiency conversion(SHG central wavelength) that is specific to the optical waveguide-typeQPM-SHG device 410. When S0 coincides with the SHG center frequency, thefundamental frequency component (1 kHz) of the Sd disappears and only adouble frequency component remains, as shown in FIG. 16B. Accordingly,by configuring a loop that integrates this wobbling signal Sd and thefeedback signal SL, which has passed through the band-pass filter 427,and supplies the integration over time as the feedback value S0 to thewavelength-variable portion 402, it is possible to operate thewavelength-variable laser 400 constantly at a wavelength in the vicinityof the maximum efficiency conversion of the optical waveguide-typeQPM-SHG device 410.

For reference, the operation of the slow loop is described usingequations. First, a sine wave of the signal Sd generated by the signalsource 429 is given by the following Equation 3, and a current Issupplied to the wavelength-variable portion 402 is normalized using thefollowing Equation 4.Sd=asin (ωt)  (Equation 3)

(a: constant)S=Sd+S 0  (Equation 4)

Here, provided that the feedback signal SL (which depends on theconversion efficiency of the optical waveguide-type QPM-SHG device 410)for a wavelength-variable current S (Is) is approximated by thefollowing Equation 5, the relationship given by the following Equation 6is obtained by substituting the above Equation 4 into the followingEquation 5.SL=−b(S−Sx)² +c  (Equation 5)

(b, c: constants) $\begin{matrix}\begin{matrix}{{S\; L} = {{- {b\left( {{S0} + {S\; d} - {S\; x}} \right)}^{2}} + c}} \\{= {{{- b}\left\{ {{S0} - {S\; x} + {a\;{\sin\left( {\omega\; t} \right)}}} \right\}^{2}} + c}}\end{matrix} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

In the above Equation 6, Sx is unknown and is a solution obtained by thefollowing probe.

The product of the above feedback signal SL and the source signal Sd iscalculated by the multiplication means 428. That is, the calculation ofthe following Equation 7 is performed. $\begin{matrix}\begin{matrix}{{S\; L \times a\;\sin\;\left( {\omega\; t} \right)}\; = {{\left( {{a\;{b\left( {{S0} - {S\; x}} \right)}^{2}} + c}\; \right)\sin\;\theta} +}} \\{{2\;{{a2b}\left( {{S0} - {S\; x}} \right)}\sin\; 2\;\theta} +} \\{a\; 3\; b\;\sin\; 3\;\theta}\end{matrix} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$ (θ=ωt)

When integrating the output signal of this product with the integrationmeans 430, since sinθ and sin3θ are odd functions, their integral is 0.Accordingly, when only the second term is calculated, the followingEquation 8 is given, and a signal that is linearly proportional to thecentral value S0 of the injection current is obtained. $\begin{matrix}\begin{matrix}{{\int{\left( {S\; L \times a\;\sin\;\theta} \right){\mathbb{d}\theta}}} = {2\;{{a2b}\left( {{S0} - {S\; x}} \right)}{\int{\sin\; 2\;\theta{\mathbb{d}\theta}}}}} \\{= {2\;\pi\; a\; 2\;{b\left( {{S0} - {S\; x}} \right)}}}\end{matrix} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

Accordingly, when a feedback loop in which this multiplication signal is0 is formed, then it is inevitable that S0=Sx, so that the optimumcurrent is constantly supplied to the wavelength-variable portion 402.

It should be noted that when the frequency of the disturbance signalsupplied from the signal source 429 is 1 kHz, it is desirable that theoperating frequency of the slow feedback loop be about one-tenth ofthat, i.e., about 100 Hz.

Seventh Embodiment

FIG. 17 is a block diagram showing the control circuit of a light sourcedevice according to a seventh embodiment of the present invention. InFIG. 17, numeral 400 denotes the wavelength-variable laser, and thiswavelength-variable laser 400 is made of the emission portion 401, thewavelength-variable portion 402 and a phase-variable portion 403.Additionally, the emission portion 401 of the wavelength-variable laser400, the spectroscopic device 421, the photodetector 422, the headamplifier 423, the differential calculation means 424, the integrationmeans 425 and the current conversion amplifier 426 constitute aso-called fast feedback loop that corrects the fluctuating portion ofthe output of the second harmonic by controlling the current injectedinto the emission portion 401. Further, the spectroscopic device 421,the photodetector 422, the head amplifier 423, the differentialcalculation means 424, the integration means 425, the band-pass filter427, the multiplication means 428, the signal source 429, theintegration means 430, the addition calculation means 431, currentconversion amplifiers 432 and 435, a coefficient multiplier 433 and thewavelength-variable portion 402 constitute a so-called slow loop thatconstantly probes the maximum point of the conversion efficiency of theoptical waveguide-type QPM-SHG device 410. The above-described fast loopand slow loop have equal functions to the fast loop and slow loop in thesixth embodiment. The feature of this embodiment lies in the provisionof structural elements for outputting a desired output and a secondharmonic of the modulation waveform when recording signals. This isdescribed in the following.

First, after switching from a reproducing mode to a recording mode(R/W→H), a recording waveform generation means 440 generates a recordingsignal SG corresponding to a recording signal stream. The recordingsignal SG shown in FIG. 18 is given based on the assumption that therecording medium is a phase change medium. That is, when the information“1” is recorded, a pulse train with the maximum power is output for theformation of an amorphous mark on the phase change medium, whereas whenthe information “0” is recorded (when information is erased), a constantvalue with a medium power is output that is supposed to locallycrystallize the phase change medium. This recording signal SG issuperimposed, via a modulator 450, on the current IL injected into theemission portion 401, and (after being converted with the opticalwaveguide-type QPM-SHG device 410) projected as modulated light onto therecording medium. While the target value of the fast loop at this timemay be a value corresponding to the desired average power of themodulated light, the modulation signal (recording signal) SG generatedby the recording waveform generation means 440 also may be used directlyas the target value (the “expected value”).

The modulation signal (recording signal) SG is further supplied, via aninversion means 441 and an equalization means 442, to the phase-variableportion 403 of the wavelength-variable laser 400. This is described inthe following. Although in the sixth embodiment the oscillationwavelength was changed only with the amount of a current injected intothe wavelength-variable portion 402 of the wavelength-variable laser400, an actual wavelength-variable laser 400 is provided with thephase-variable portion 403 for phase matching, in order to avoid modehopping when the wavelength is changed. That is, at the same time whenthe current Is is injected into the wavelength-variable portion 402, acurrent Ip that is a constant (k) multiple of this current is injectedinto the phase-variable portion 403. The phase-variable portion 403 isactually a so-called heater, which serves to change the refractive indexof the waveguide of the laser by using the heat generated by theinjection current.

Here, when the reproducing operation is switched to the recordingoperation, the current injected into the emission portion 401 isinevitably increased in order to increase the power of the laser, andthe temperature of the emission portion 401 is increased by the heatgenerated at this time. Since the emission portion 401 also has awaveguide structure, the change in the refractive index due to thegenerated heat causes the shifting of the phase as well as of thewavelength. This results in a decrease in the conversion efficiency ofthe optical waveguide-type QPM-SHG device 410 and the occurrence of modehopping. To prevent this, a current that is substantially in anti-phasewith the one injected into the emission portion 401 is supplied, via theaddition calculation means 434, to the phase-variable portion 403 inthis embodiment. More specifically, an inverted waveform Si (FIG. 17) isfirst generated with the inversion means 441, and the compensation valueSe thus obtained is converted into a current with the equalization means442. Consequently, the current injected into the emission portion 401 isincreased, while the current injected into the phase-variable portion(heater) 403 is decreased, so that the overall amount of heat that isgenerated is maintained constant during both recording and reproducing.

However, the experiments carried out by the present inventors haveproved that overcorrection occurs when the inverted waveform Si issupplied continuously directly to the phase-variable portion 403. Thisis believed to be related to the propagation and accumulation of heatinside the laser. Therefore, the inventors investigated converting theinverted waveform Si into a waveform (Se) as shown in FIG. 18 using theequalization means 442, that is, a waveform where the amount ofcorrection is set to a maximum immediately after changing the power, andthen gradually decreased by about 10% per 100 μs. It was found that thisallowed thermal correction to be performed effectively. Specifically,the equalization means 442 can be embodied by using a band-pass filteras shown in FIG. 19, for example.

In addition, the thermal correction means made of the inversion means441, the equalization means 442 and the like is a so-called open-loop,whose precision of correction is limited. In other words, there is aminor thermal correction residual. This results in the occurrence of aminor shift of the wavelength, which however is followed up by the fastfeedback loop at 100 kHz, that is, with a response of about 10 μs, sothat the output of the second harmonic itself is not affected. Here,since the decrease in the conversion efficiency of the opticalwaveguide-type QPM-SHG device 410 due to the wavelength shift iscompensated by the emission power of the source oscillation, the fastfeedback control is performed, resulting in some temporary increase inthe current IL injected into the emission portion 401; however, the slowloop eventually probes the maximum efficiency.

As described above, according to this embodiment, thermal equalizationand compensation means, a fast loop and a slow loop are successivelyemployed, thereby making it possible to effectively absorb thefluctuation in a second harmonic caused by the wavelength shift whenswitching the power from reproduction to recording.

Additionally, the sixth and seventh embodiments described the operationafter each of the feedback loops started to operate (the so-called“pull-in”); however, the following problems occur if both of thefeedback loops are operated simultaneously before the pull-in.Ordinarily, in order for a target value and a variable to be pulled instably to the operation point of a feedback loop, they need to be in apredetermined range (capture range) immediately before the pull-in. Asdescribed above, the wavelength dependence of the optical waveguide-typeQPM-SHG device is usually conspicuous, and the conversion efficiency ofthe optical waveguide-type QPM-SHG device is substantially zero whenthere is a large error, for example, a shift of about 1 nm, between theoscillation wavelength of the laser and the SHG central wavelength.Accordingly, no second harmonic is generated, and the amount of lightdetected by a light-receiving device is also zero. If the fast feedbackloop is operated at this time, the current injected into the emissionportion of the wavelength-variable laser is increased without limit inorder to make the amount of the received light to be the predeterminedtarget value. Then, if this state is allowed to continue, the laserinevitably will be damaged.

Therefore, before the pull-in, it is necessary either to suspend thefunction of the fast feedback loop or reduce the response speed to anextremely low level. For example, a sequence should be configured suchthat a constant current is passed through the emission portion of thewavelength-variable laser before the pull-in and the fast feedback loopis operated (closed) after the slow feedback loop has caused thewavelength of the laser to reach the vicinity of the wavelength of themaximum efficiency. Additionally, it is desirable that the amplitude ofthe disturbance signal in the slow feedback loop be set larger than inthe normal state, in order to broaden the capture range. For example, ifthe amplitude of the disturbance signal in the normal state is 0.1 nm interms of wavelength, it is desirable that the amplitude be set to about1 nm at the time of the pull-in. The reason is that the error betweenthe oscillation wavelength of the wavelength-variable laser and the SHGcentral wavelength is substantially within the range of 1 nm in adefault state.

INDUSTRIAL APPLICABILITY

As described above, the present invention can achieve a stable output ofa harmonic even if there occurs a change in the ambient temperature orfluctuation in the output power, so that it can be utilized for compactshort-wavelength light sources used for realizing high density foroptical disks and high resolution for displays.

1. A light source device comprising: a semiconductor laser source; asecond harmonic generation device for generating a second harmonic fromlight emitted from the semiconductor laser source; a means forcontrolling output light power of the semiconductor laser source in sucha manner that a power of the second harmonic emitted from the secondharmonic generation device is constant; a means for controlling awavelength of light emitted from the semiconductor laser source; a meansfor periodically changing the wavelength of light emitted from thesemiconductor laser source; and a means for detecting a change in outputlight power of the semiconductor laser source or a change in an amountof a current injected into the semiconductor laser source that occurswhen the wavelength of light emitted from the semiconductor laser sourceis periodically changed, wherein the wavelength of light emitted fromthe semiconductor laser source is controlled to an optimum wavelength ofthe second harmonic generation device based on the change in outputlight power of the semiconductor laser source or the change in theamount of the current injected into the semiconductor laser source thatoccurs when the wavelength of light emitted from the semiconductor lasersource is periodically changed.
 2. The light source device according toclaim 1, wherein the semiconductor laser source is a distributed Braggreflector (DBR) semiconductor laser source having a DBR region.
 3. Thelight source device according to claim 2, wherein the semiconductorlaser source has a phase adjustment region.
 4. The light source deviceaccording to claim 3, wherein the wavelength of light emitted from thesemiconductor laser source is changed by changing the amount of thecurrent injected into the phase adjustment region of the semiconductorlaser source.
 5. The light source device according to claim 3, whereinthe wavelength of light emitted from the semiconductor laser source ischanged by changing, at a constant ratio, an amount of a currentinjected into the phase control region and an amount of a currentinjected into an active layer region of the semiconductor laser source.6. The light source device according to claim 1, wherein a heater isprovided in the vicinity of the semiconductor laser source, and thewavelength of light emitted from the semiconductor laser source ischanged by changing an amount of heat applied by the heater.
 7. Anoptical information recording/reproducing apparatus comprising: thelight source device according to claim 1; a condensing optical systemfor guiding light from the light source device to an informationcarrier; and a means for detecting light reflected from the informationcarrier.
 8. A light source device comprising: a semiconductor lasersource; a second harmonic generation device for generating a secondharmonic from light emitted from the semiconductor laser source; a meansfor controlling output light power of the semiconductor laser source insuch a manner that a power of the second harmonic emitted from thesecond harmonic generation device is constant; a means for controllingan optimum wavelength of the second harmonic generation device; a meansfor periodically changing a wavelength of light emitted from thesemiconductor laser source; and a means for detecting a change in outputlight power of the semiconductor laser source or a change in an amountof a current injected into the semiconductor laser source that occurswhen the wavelength of light emitted from the semiconductor laser sourceis periodically changed, wherein a wavelength of the second harmonicgeneration device is controlled to the optimum wavelength based on thechange in output light power of the semiconductor laser source or thechange in the amount of the current injected into the semiconductorlaser source that occurs when the wavelength of the semiconductor lasersource is periodically changed.
 9. The light source device according toclaim 8, wherein the semiconductor laser source is a distributed Braggreflector (DBR) semiconductor laser source having a DBR region.
 10. Thelight source device according to claim 9, wherein the semiconductorlaser source has a phase adjustment region.
 11. The light source deviceaccording to claim 10, wherein the wavelength of light emitted from thesemiconductor laser source is changed by changing the amount of thecurrent injected into the phase adjustment region of the semiconductorlaser source.
 12. The light source device according to claim 10, whereinthe wavelength of light emitted from the semiconductor laser source ischanged by changing, at a constant ratio, an amount of a currentinjected into the phase control region and an amount of a currentinjected into an active layer region of the semiconductor laser source.13. The light source device according to claim 8, wherein a heater isprovided in the vicinity of the semiconductor laser source, and thewavelength of light emitted from the semiconductor laser source ischanged by changing an amount of heat applied by the heater.
 14. Thelight source device according to claim 8, wherein a heater is providedin the vicinity of an optical waveguide of the second harmonicgeneration device.
 15. An optical information recording/reproducingapparatus comprising: the light source device according to claim 8; acondensing optical system for guiding light from the light source deviceto an information carrier; and a means for detecting light reflectedfrom the information carrier.